1. Field of the Invention
The present invention relates to an optical package arrangement and, more particularly, to such a package which is capable of reducing internal reflections while maintaining all components within the package along a common axis.
2. Description of the Prior Art
In most optical communication systems, it is desirable to prevent reflected signals from re-entering the transmitting device. For example, distributed feedback (DFB) lasers, utilized in high bit rate systems where single mode operation is required, are known to be extremely sensitive to reflected signals. In order to reduce optical reflections, optical isolators are often utlized. One such isolator arrangement being disclosed in U.S. Pat. No. 4,548,478 issued to Shirasaki et al. on Oct. 22, 1985. Although these isolators are able to reduce reflected signals orginating at the output, reflections from the front face of the isolator, or any other elements (windows, filters, etc.) inserted between the transmitting device and the isolator, may still re-enter the transmitting device and cause a degradation in the system performance.
Another approach to the reflection problem is to bevel or tilt various optical elements or surfaces within the package so as to redirect any reflected signal away from the transmitting device. The use of tilts and bevels usually requires that the elements be assembled in an off-axis arrangement. One such arrangement is described in a paper entitled "Destributed-feedback laser diode module with a novel and compact optical isolator for gigabit optical transmission systems" by T. Chikama et al. appearing in the OFC Proceedings 1986, Paper No. ME4. FIG. 1 of Chikama et al. paper clearly illustrates the displacement between a DFB laser diode chip and the single mode optical fiber. The loss of cylindrical symmetry between the laser diode and the fiber is considered to adversely impact both the cost and performance of the packaged arrangement. In particular, the eccentricity of this off-axis arrangement necessitates the use of eccentric packaging which must be both rotationally oriented and radially aligned. That is, any ferrule or sleeve component used to attach the off-set fiber to the package must include an off-set bore to accomodate the fiber. Therfore, the ferrule must be oriented and aligned with a high degree of accuracy to ensure alignment of the fiber to the off-axis beam. In contrast, for an on-axis arrangement, a fiber ferrule may be rotated in any manner about the optical axis without affecting the alignment. Clearly, an on-axis arrangement is the preferred alternative.
One prior art technique for maintaining an on-axis alignment while utilizing a beveled fiber to reduce reflections is described in an article entitled "General purpose single-mode laser package provided with a parallel beam output having -60 dB interface feedback", by C. K. Wong et al. appearing the the Proceedings of ECOCp. 215-8. As illustrated in FIG. 2C of Wong et al., a modified fiber ferrule is used to hold a beveled fiber at the angle which maximizes coupling of the beam and fiber. In particular, a fiber ferrule is machined to include an eccentric bore such that the combination of the ferrule eccentricity and the fiber bevel allows the output signal to be on-axis. Although the Wong et al. arrangement provides for the output signal to be on-axis, the use of an eccentric fiber ferrule raises the same radial alignment difficulties as discussed above.
Therefore, a need remains in the art for an on-axis optical arrangement which is capable of preventing reflections while maintaining cylindrical symmetry such that radial alignment problems are avoided.